This widget is far from finished, but it's off to a good start.
Also added a GResizeEvent and GWidget::resize_event() so that widgets
can react to being resized.
Previously we'd blit every pixel in every window that intersected any dirty
rect to the back buffer. With this patch, we limit ourselves to blitting the
pixels inside the actual dirty rects.
There's still a lot of optimizations to make in this code.
